Electro-optic modulator with inverse tapered waveguides

ABSTRACT

An integrated circuit that includes an optical waveguide to convey an optical signal via an optical mode in an on-chip optical waveguide is described. In this integrated circuit, a cross-sectional area of the optical waveguide may be tapered in proximity to an electro-optic modulator in the integrated circuit, such as a germanium electro-optic modulator or a quantum-well (QW) electro-optic modulator. In particular, the cross-sectional area may be tapered from a first diameter distal from the electro-optic modulator to a second diameter proximate to the electro-optic modulator. This so-called ‘inverse taper’ may increase the spatial extent or size of the optical mode, thereby allowing the optical signal to be optically coupled to or from the electro-optic modulator with low optical loss.

GOVERNMENT LICENSE RIGHTS

The United States Government has a paid-up license in this invention andthe right in limited circumstances to require the patent owner tolicense others on reasonable terms as provided for by the terms ofAgreement No. HR0011-08-9-0001 awarded by the Defense Advanced ResearchProjects Administration.

BACKGROUND

1. Field

The present disclosure generally relates to electro-optic-modulatorcircuits. More specifically, the present disclosure relates to anelectro-optic-modulator circuit that includes an inverse taperedwaveguide.

2. Related Art

Silicon photonics is a promising new technology that can providelow-power, high-bandwidth and low latency-interconnects in futurecomputing systems. However, in order to implement silicon photoniclinks, efficient light modulators are needed. It is complicated toconstruct efficient light modulators because the electro-optic effect insilicon (Si) is weak. As a consequence, a number of different ofmodulation mechanisms are being investigated. Two promising modulationmechanisms are the electro-absorption associated with thequantum-confined stark effect (QCSE) in SiGe/Ge quantum-well (QW)devices, and the electro-absorption associated with the Franz-Keldysh(FK) effect in tensile-strained germanium (Ge).

QCSE provides a strong electro-absorption mechanism, and has been usedto make high-speed, low-power and compact opto-electronic devices usingIII-V materials. In practice, electro-absorption associated with theQCSE in a multiple QW structure that includes germanium QWs which areseparated by silicon-germanium barriers can offer a much strongerelectro-optic effect than a depletion-based silicon light modulator.Consequently, silicon-germanium QCSE devices can provide broadbandoperation with low driver voltage. In addition, the same QCSE device canbe used as either a light modulator or a photo detector.

Similarly, increased electro-absorption (relative to silicon) can alsobe achieved using the FK effect in Ge_(1-x)Si_(x) (for example, usingthe enhanced FK effect in tensile strained, epitaxialgermanium-on-silicon). Because the FK effect takes place on asub-pico-second time scale, the speed of the electro-absorptionmechanism based on the FK effect is only limited by the RC delay, andcan be designed to achieve very high bandwidth. Moreover, the sameFK-effect device can also be used as a photo detector with highresponsivity and high bandwidth.

However, it is very challenging to integrate these light modulators withsilicon-based optical waveguides, which makes it hard to use these lightmodulators. In particular, it is very challenging to fabricateelectro-absorption light modulators with sub-micron on-chip siliconoptical waveguides because epitaxial growth is needed for the activematerial layers in the electro-absorption light modulators, such as themultilayer QW structures or the tensile-strained germanium layer. Thisepitaxial growth is in a direction normal to the substrate, while thesilicon optical waveguides carrying the optical signals are normally inthe plane of the substrate. Moreover, it is difficult to couple thelight from a sub-micron silicon optical waveguide to the active materiallayers to modulate the light, and then to couple the modulated lightback to a sub-micron output silicon optical waveguide with low opticalloss.

Hence, what is needed is an electro-optic-modulator circuit that doesnot suffer from the above-described problems.

SUMMARY

One embodiment of the present disclosure provides an integrated circuitthat includes a substrate. Disposed on a surface of the substrate, theintegrated circuit includes a first optical waveguide having a firstend, and a second optical waveguide having a second end. Moreover, anelectro-optic modulator is positioned between and is mechanicallycoupled to the first end and the second end. Note that a cross-sectionalarea of a given optical waveguide (which can include the first opticalwaveguide or the second optical waveguide) is tapered from a firstdiameter distal from a given end (which can include the first end or thesecond end) to a second, smaller diameter proximate to the given end,thereby facilitating optical coupling between the given opticalwaveguide and the electro-optic modulator.

In some embodiments, the substrate includes a semiconductor, such assilicon. Moreover, the taper of the given optical waveguide may be overa length of a region of the given optical waveguide. For example, thelength of the region may be between 10 and 200 μm. In addition, thefirst diameter of the given optical waveguide may be less than 1 μm, andthe second diameter of the given optical waveguide may be less than 0.25μm.

Furthermore, the electro-optic modulator may include a germaniumelectro-optic modulator or a quantum-well (QW) electro-optic modulator.Note that the QW electro-optic modulator may include alternating layersof silicon and silicon-germanium. Additionally, the integrated circuitmay include an underlayer between the surface of the substrate and theQW electro-optic modulator. This underlayer may include germanium.

In some embodiments, the integrated circuit includes an optical-couplingmaterial between the given end and a side of the electro-opticmodulator.

Another embodiment provides a system that includes the integratedcircuit.

Another embodiment provides a method for selectively opticallymodulating an optical signal in an integrated circuit. During operation,the integrated circuit conveys the optical signal using an optical modein an optical waveguide, where the optical waveguide has an end and isdisposed on a surface of a substrate. Then, the integrated circuitincreases a spatial extent of the optical mode in proximity to the endusing a taper of a cross-sectional area of the optical waveguide.Moreover, the integrated circuit optically couples the optical signal toan electro-optic modulator that is disposed on the surface of thesubstrate, and which is mechanically coupled to the end. Next, theintegrated circuit selectively optically modulates the optical signal inthe electro-optic modulator based on an electrical signal.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block diagram illustrating a side view of an integratedcircuit in accordance with an embodiment of the present disclosure.

FIG. 2 is a block diagram illustrating a top view of the integratedcircuit in FIG. 1 in accordance with an embodiment of the presentdisclosure.

FIG. 3 is a block diagram illustrating a system that includes theintegrated circuit of FIGS. 1 and 2 in accordance with an embodiment ofthe present disclosure.

FIG. 4 is a flow chart illustrating a process for selectively opticallymodulating an optical signal in the integrated circuit of FIGS. 1 and 2in accordance with an embodiment of the present disclosure.

Note that like reference numerals refer to corresponding partsthroughout the drawings. Moreover, multiple instances of the same partare designated by a common prefix separated from an instance number by adash.

DETAILED DESCRIPTION

Embodiments of an integrated circuit, a system that includes theintegrated circuit, and a method for selectively optically modulating anoptical signal in the integrated circuit are described. This integratedcircuit includes an on-chip optical waveguide to convey an opticalsignal via an optical mode in the optical waveguide. Moreover, across-sectional area of the optical waveguide may be tapered inproximity to an electro-optic modulator in the integrated circuit, suchas a germanium electro-optic modulator or a quantum-well (QW)electro-optic modulator. In particular, the cross-sectional area may betapered from a first diameter distal from the electro-optic modulator toa second diameter proximate to the electro-optic modulator. Thisso-called ‘inverse taper’ may increase the spatial extent or size of theoptical mode, thereby allowing the optical signal to be opticallycoupled to or from the electro-optic modulator with low optical loss.Consequently, the integrated circuit may solve the problem ofintegrating the electro-optic modulator with on-chip, sub-micron opticalwaveguides.

While a wide variety of materials can be used as a substrate in theintegrated circuit (such as a semiconductor, glass or plastic), in thediscussion that follows silicon is used as an illustrative example.

We now describe embodiments of the integrated circuit. FIG. 1 presents ablock diagram illustrating a side view of an integrated circuit 100, andFIG. 2 presents a block diagram illustrating a top view of integratedcircuit 100. This integrated circuit includes a substrate 110 (such assilicon). Disposed on a surface 112 of substrate 110, integrated circuit100 includes an optical waveguide 114-1 having an end 116-1, and anoptical waveguide 114-2 having an end 116-2. These optical waveguidesmay convey an optical signal (i.e., light) having wavelengths between1.1-1.7 μm, such as an optical signal having a fundamental wavelength of1.3 or 1.55 μm.

Moreover, an electro-optic modulator 118 is positioned between andmechanically coupled to ends 116. This electro-optic modulator may beturned ‘on’ or ‘off’ (i.e., may block or pass the light) based on anapplied voltage that is controlled by control logic 132.

Note that a cross-sectional area of a given optical waveguide (which caninclude optical waveguide 114-1 or optical waveguide 114-2) is taperedfrom a diameter 120-1 distal from a given end (which can include end116-1 or end 116-2) to a smaller diameter 120-2 proximate to the givenend, thereby facilitating optical coupling between the given opticalwaveguide and electro-optic modulator 118.

Furthermore, the taper of the given optical waveguide may be over alength (such as one of lengths 122) of a region of the given opticalwaveguide. For example, each of lengths 122 may be between 10 and 200μm. In addition, diameter 120-1 of the given optical waveguide may beless than 1 μm, and diameter 120-2 of the given optical waveguide may beless than 0.25 μm. For example, each of lengths 122 may be 100 μm,diameter 120-1 may be 0.5 μm, and diameter 120-2 may be 100 nm.

Note that electro-optic modulator 118 may include a germaniumelectro-optic modulator or a QW electro-optic modulator that provideselectro-absorption using the quantum-confined stark effect (QCSE). Notethat the QW electro-optic modulator may include alternating layers ofsilicon and silicon-germanium as the active electro-absorption material.When SiGe/Ge QWs are grown on crystal silicon, a silicon-germaniumbuffer layer typically needs to be grown first to gradually relax thelattice mismatch between the crystal structures of germanium andsilicon.

One possible way to fabricate a QCSE QW electro-optic modulator on asilicon-on-insulator (SOI) substrate with sub-micron optical waveguidesis to use a selective growth technique in which electro-optic modulator118 is grown in a ‘hole’ in optical waveguides 114. For example, awindow may be opened up on an SOI substrate with sub-micron opticalwaveguides by etching away the optical waveguide and the buried oxide.Then, using selective epitaxial growth, the QCSE QW electro-opticmodulator can be fabricated in the hole, starting with an optionalbuffer or underlayer 124 on surface 112, followed by the SiGe/Ge QWs,and then a cap layer. For example, this underlayer may include puregermanium. Note that optional underlayer 124 may extend under opticalwaveguides 114 or may only be located under electro-optic modulator 118.

Note that the thickness of optional underlayer 124 and the QWs may becontrolled such that the center of the QWs is aligned with the center ofthe optical-waveguide optical mode (thereby reducing or eliminatingoptical absorption in optional underlayer 124). In this way, light fromthe input optical waveguide (such as optical waveguide 114-1) isbutt-coupled to the region with the QCSE QWs, where it is modulated andthen butt-coupled back to the output optical waveguide (such as opticalwaveguide 114-2).

One problem with this approach is that, in the region with the QCSE QWs,light is weakly guided in the vertical direction, whereas in opticalwaveguides 114 it is highly confined due to the big difference betweenthe index or refraction of silicon and silicon dioxide. Stateddifferently, the spatial extent of the optical mode in the sub-micronsilicon optical waveguides is significantly smaller than the spatialextent of the optical mode in the region with the QCSE QWs. As aconsequence, significant optical-coupling loss can occur at both of ends116. For example, if a sub-micron optical waveguide has an optical-modesize of 0.25 μm, and the region with the QCSE QWs has an optical-modesize of 2 μm in the vertical direction, there will be more than 10 dBoptical-coupling loss at each of ends 116. A modulator with such highinsertion loss is useless.

Integrated circuit 100 reduces or eliminates this optical-mode mismatchproblem using inverse nano-tapers of optical waveguides 114. Inparticular, instead of terminating the sub-micron optical waveguidedirected at the interface with the region with the QCSE QWs (i.e., atone of ends 116 in proximity to electro-optic modulator 118), inputoptical waveguide 114-1 and output optical waveguide 114-2 each have anano-tip in which the inverse tapers terminate at respective ends 116.These inverse nano-tapers expand the spatial extent of the optical mode.With proper design of the taper, the optical-mode size at ends 116 canmatch that of electro-optic modulator 118. Therefore, lowoptical-coupling loss can be achieved when coupling light in and out ofthe region with the QCSE QWs.

In another embodiment, epitaxial Ge_(1-x)Si_(x) is used as activeelectro-absorption material in electro-optic modulator 118 using theFranz-Keldysh (FK) effect. Instead of growing SiGe/Ge QWs,Ge_(1-x)Si_(x) material may be grown between the two cap layers in thehole. Once again, the inverse nano-tapers of optical waveguides 114 maybe used to optically couple the light in and out of the region with theactive Ge_(1-x)Si_(x) material in electro-optic modulator 118 with lowoptical-coupling loss. For example, bulk germanium may be used. Thisgrows directly onto silicon, and has a high index of refraction, but hasweaker electro-optic modulation than the QCSE QWs. Better contrast orextinction (i.e., a better on/off modulation ratio) can be obtained byincreasing length 128 of electro-optic modulator 118, but the averagepower may be decreased. Alternatively or additionally, an optionaloptical-gain stage 130 may be used in integrated circuit 100.

Note that the optical coupling may be increased by decreasing thesurface roughness (and, thus, the scattering loss) of ends 116.Alternatively, the spatial extent of the optical mode may be increasedusing the inverse nano-taper so that the light doesn't ‘see’ ends 116.However, a larger optical-mode size increases the capacitance, and thusthe modulation energy. For this reason, there may be an upper bound onthe modulated volume in electro-optic modulator 118.

Additionally, note that the layers grown in the hole may have apyramidal shape (i.e., there may be a wall angle associated with thegrowth). To address this, the layers may be overgrown and polished backto obtain a desired total thickness of the active region inelectro-optic modulator 118. For example, a dry etch may be used toobtain vertical walls proximate to ends 116. In some embodiments,integrated circuit 100 includes an optional optical-coupling material126 between the given end and a side of electro-optic modulator 118 tofill in any resulting space(s). Ideally, optional optical-couplingmaterial 126 material has an index of refraction between that of siliconand the material(s) in electro-optic modulator 118. For example,optional optical-coupling material 126 may include silicon nitride orsilicon dioxide.

In exemplary embodiments, length 128 of a germanium electro-opticmodulator is 50 μm, and length 128 of a QW electro-optic modulator is 25μm. Furthermore, there may be 30-60 periods of alternating SiGe/Gelayers in the QW electro-optic modulator. For example, there may be 20periods. Note that the multiple QW active structure may have agermanium-layer thickness of 135 Å as the QW layer, and a Ge₂₀Si₈₀-layerthickness of 65 Å as the barrier layer.

Moreover, the multiple QW active structure may be fabricated in the holeas follows. Starting with the silicon substrate and continuing up to thetop silicon-on-insulator (SOI) layer: a 0.25-μm thick Ge₉₀Si₁₀ layerwith p-type doping (1e18 cm⁻³) using boron may be fabricated. Then, themultiple QWs may be fabricated with a total thickness of 0.4 μm and nodoping. Next, a 0.4-μm thick Ge₉₀Si₁₀ layer with n-type doping (1e18cm⁻³) using phosphorous and/or arsenic may be fabricated.

Note that the p-type germanium-silicon layer (i.e., optional underlayer124) may provide a thin buffer layer to relax strain and to generate anew lattice constant for the layers in the multiple QWs. Moreover, thep-type germanium-silicon layer may provide a contact window tofacilitate an electric field normal to the plane of the multiple QWs.The thickness of this layer may be selected to enhance the amplitude ofthe optical field associated with the optical mode in the multiple QWlayers so that the amplitude in optional underlayer 124 and in thetop-contact or cap layer is reduced. This n-type cap layer provides thecontact window to initiate electric-field lines of force that are closedby the bottom p-type contact window (i.e., optional underlayer 124).Furthermore, the silicon-alloy content in the contact windows may bechosen to minimize the lattice mismatch between the layers in themultiple QWs and the lattice constant of optional underlayer 124. Thus,the new lattice constant may be uniform in the active-layer design, andmay be set to the relaxed lattice constant of optional underlayer 124.

Integrated circuit 100 may be used in a variety of applications. This isshown in FIG. 3, which presents a block diagram illustrating a system300 that includes the integrated circuit 100. System 300 may includes: aVLSI circuit, a switch, a hub, a bridge, a router, a communicationsystem, a storage area network, a data center, a network (such as alocal area network), and/or a computer system (such as a multiple-coreprocessor computer system). Furthermore, the computer system mayinclude, but is not limited to: a server (such as a multi-socket,multi-rack server), a laptop computer, a communication device or system,a personal computer, a work station, a mainframe computer, a blade, anenterprise computer, a data center, a portable-computing device, asupercomputer, a network-attached-storage (NAS) system, astorage-area-network (SAN) system, and/or another electronic computingdevice. Note that a given computer system may be at one location or maybe distributed over multiple, geographically dispersed locations.

Integrated circuit 100 (FIGS. 1 and 2), as well as system 300, mayinclude fewer components or additional components. Although integratedcircuit 100 (FIGS. 1 and 2), as well as system 300, are illustrated ashaving a number of discrete items, these circuits and devices areintended to be functional descriptions of the various features that maybe present rather than structural schematics of the embodimentsdescribed herein. Consequently, in these embodiments two or morecomponents may be combined into a single component, and/or a position ofone or more components may be changed. In addition, functionality in thepreceding embodiments of the integrated circuit and system may beimplemented more in hardware and less in software, or less in hardwareand more in software, as is known in the art. For example, functionalitymay be implemented in one or more application-specific integratedcircuits (ASICs) and/or one or more digital signal processors (DSPs).

While the preceding embodiments have been illustrated with particularelements and compounds, a wide variety of materials and compositions(including stoichiometric and non-stoichiometric compositions) may beused, as is known to one of skill in the art. Furthermore, thesematerials and compounds may be fabricated using a wide variety ofprocessing techniques, including evaporation, sputtering, molecular-beamepitaxy, wet or dry etching (such as photolithography or direct-writelithography), polishing, etc.

We now describe embodiments of a process. FIG. 4 presents a flow chartillustrating a process 400 for selectively optically modulating anoptical signal in integrated circuit 100 (FIGS. 1 and 2). Duringoperation, the integrated circuit conveys the optical signal using anoptical mode in an optical waveguide (operation 410), where the opticalwaveguide has an end and is disposed on a surface of a substrate. Then,the integrated circuit increases a spatial extent of the optical mode inproximity to the end using a taper of a cross-sectional area of theoptical waveguide (operation 412). Moreover, the integrated circuitoptically couples the optical signal to an electro-optic modulator thatis disposed on the surface of the substrate, and which is mechanicallycoupled to the end (operation 414). Next, the integrated circuitselectively optically modulates the optical signal in the electro-opticmodulator based on an electrical signal (operation 416).

In some embodiments of process 400, there are additional or feweroperations. Moreover, the order of the operations may be changed, and/ortwo or more operations may be combined into a single operation.

The foregoing description is intended to enable any person skilled inthe art to make and use the disclosure, and is provided in the contextof a particular application and its requirements. Moreover, theforegoing descriptions of embodiments of the present disclosure havebeen presented for purposes of illustration and description only. Theyare not intended to be exhaustive or to limit the present disclosure tothe forms disclosed. Accordingly, many modifications and variations willbe apparent to practitioners skilled in the art, and the generalprinciples defined herein may be applied to other embodiments andapplications without departing from the spirit and scope of the presentdisclosure. Additionally, the discussion of the preceding embodiments isnot intended to limit the present disclosure. Thus, the presentdisclosure is not intended to be limited to the embodiments shown, butis to be accorded the widest scope consistent with the principles andfeatures disclosed herein.

1. An integrated circuit, comprising a substrate, wherein disposed on asurface of the substrate, the integrated circuit includes: a firstoptical waveguide having a first end; a second optical waveguide havinga second end; an electro-optic modulator positioned between andmechanically butt-coupled to the first end and the second end, wherein across-sectional area of a given optical waveguide, which can include thefirst optical waveguide or the second optical waveguide, is tapered froma first diameter distal from a given end, which can include the firstend or the second end, to a second, smaller diameter proximate to thegiven end, thereby facilitating optical coupling between the givenoptical waveguide and the electro-optic modulator; and an underlayerbetween the surface of the substrate and the electro-optic modulator. 2.The integrated circuit of claim 1, wherein the substrate includes asemiconductor.
 3. The integrated circuit of claim 2, wherein thesemiconductor includes silicon.
 4. The integrated circuit of claim 1,wherein the taper of the given optical waveguide is over a length of aregion of the given optical waveguide.
 5. The integrated circuit ofclaim 4, wherein the length is between 10 and 200 μm.
 6. The integratedcircuit of claim 1, wherein the electro-optic modulator includes agermanium electro-optic modulator.
 7. The integrated circuit of claim 1,wherein the electro-optic modulator includes a quantum-well (QW)electro-optic modulator.
 8. The integrated circuit of claim 7, whereinthe QW electro-optic modulator includes alternating layers of siliconand silicon-germanium.
 9. The integrated circuit of claim 1, wherein theunderlayer includes germanium.
 10. The integrated circuit of claim 1,wherein the first diameter of the given optical waveguide is less than 1μm; and wherein the second diameter of the given optical waveguide isless than 0.25 μm.
 11. The integrated circuit of claim 1, wherein theintegrated circuit further includes an optical-coupling material betweenthe given end and a side of the electro-optic modulator.
 12. A system,comprising: a processor; and an integrated circuit, wherein theintegrated circuit includes a substrate, and wherein disposed on asurface of the substrate, the integrated circuit includes: a firstoptical waveguide having a first end; a second optical waveguide havinga second end; an electro-optic modulator positioned between andmechanically butt-coupled to the first end and the second end, wherein across-sectional area of a given optical waveguide, which can include thefirst optical waveguide or the second optical waveguide, is tapered froma first diameter distal from a given end, which can include the firstend or the second end, to a second, smaller diameter proximate to thegiven end, thereby facilitating optical coupling between the givenoptical waveguide and the electro-optic modulator; and an underlayerbetween the surface of the substrate and the electro-optic modulator.13. The system of claim 12, wherein the substrate includes asemiconductor.
 14. The system of claim 12, wherein the taper of thegiven optical waveguide is over a length of a region of the givenoptical waveguide.
 15. The system of claim 12, wherein the electro-opticmodulator includes a germanium electro-optic modulator.
 16. The systemof claim 12, wherein the electro-optic modulator includes a QWelectro-optic modulator.
 17. The system of claim 16, wherein the QWelectro-optic modulator includes alternating layers of silicon andsilicon-germanium.
 18. A method for selectively optically modulating anoptical signal in an integrated circuit, comprising: conveying theoptical signal using an optical mode in an optical waveguide, whereinthe optical waveguide has an end and is disposed on a surface of asubstrate; increasing a spatial extent of the optical mode in proximityto the end using a taper of a cross-sectional area of the opticalwaveguide; optically coupling the optical signal to an electro-opticmodulator that is disposed on the surface of the substrate, and which ismechanically butt-coupled to the end, wherein an underlayer is locatedbetween the surface of the substrate and the electro-optic modulator;and selectively optically modulating the optical signal in theelectro-optic modulator based on an electrical signal.